Studies of chemical metabolism and disposition are designed to provide both applied knowledge in support of chronic toxicity tests conducted by the National Toxicology Program and basic knowledge of those chemical structure and property relationships which determine toxicity. Studies of methyl carbamate metabolism and disposition in rats and mice indicate that a species dependent variation in sensitivity to toxicity and carcinogenicity induced by this compound may be attributable to a much greater capacity of mice to metabolize and clear methyl carbamate. Studies of ethyl carbamate for structure activity relation to methyl carbamate indicate structurally related variations in toxicity may be attributable to subtle differences in kinetics of metabolism by mice.